Under conditions where a vehicle operator indicates a desire for increased vehicle acceleration by pressing on an accelerator pedal, transmission assembly input torque may have to be increased to achieve the desired increase in vehicle acceleration. However, if the desired transmission assembly input torque requested exceeds a current transmission assembly input torque limit, then any excess transmission assembly input torque may only accelerate the transmission assembly input speed, result in a large clutch slip for a vehicle transmission with one or more transmission clutches, and may not result in an increase in the vehicle speed. In such a case, in addition to not achieving the driver's demand for an increase in vehicle acceleration, the resulting clutch slip may result in durability issues.
One example where a vehicle transmission may have a torque limit includes a dual clutch transmission (DCT) in a clutch torque tracking mode. More specifically, a torque tracking mode may include conditions where a clutch torque capacity of an active clutch of the DCT is set to be above the transmission assembly input torque by a threshold amount. In other words, the active clutch may not be locked at a maximum torque capacity possible, but may be maintained below the transmission assembly input torque threshold. Benefits of torque tracking may include faster clutch opening responsive time, and less hydraulic pressure demand from a pump configured to provide hydraulic fluid to the DCT clutches. However, in such a torque tracking mode, any desired increase in transmission assembly input torque must be coordinated with an increase in clutch toque capacity. Typically, some sort of rate limit may be imposed on the increase in transmission assembly input torque to avoid transmission assembly input torque increasing beyond clutch torque capacity.
Another example may comprise a shift event for a dual clutch transmission. Typically, during an upshift event, a clutch torque capacity may be lowered on an off-going clutch, while clutch torque capacity may be simultaneously increased on an on-coming clutch. The shift may finish, or conclude, when transmission input speed decreases to a speed determined by the new gear ratio, with the on-coming clutch carrying all of the transmission input torque.
If, during such an upshift event, a vehicle operator suddenly steps into the accelerator pedal, an increase in transmission input torque may be needed to accomplish the vehicle operator's request for more acceleration. However, the problem is that clutch torque capacity for such a shift may be scheduled based on the transmission assembly input torque requested at the start of the shift. If the transmission input torque rises too quickly during a shift, the clutch may not be able to increase torque capacity as quickly, and once the transmission input torque exceeds the clutch torque capacity, the transmission assembly input speed may start to accelerate and the shift may not finish because the upshift necessitates a decrease in the transmission assembly input speed.
A vehicle may solve this problem by limiting the increase in the transmission assembly input torque as a function of clutch torque capacity, where the transmission assembly input torque is limited below driver requested torque and increased slowly in coordination with the increasing clutch torque capacity. This may result in a slower response to the driver's request for more vehicle acceleration, which may be perceived by the vehicle operator as hesitation. Furthermore, the engine may need additional time t0 build torque responsive to the clutch torque limit being removed because of the response time of certain actuators (e.g. turbo delay, etc.).
Another example may include a situation where a clutch torque capacity may be incorrectly estimated to be large enough for an increase in transmission assembly input torque, but where in fact the clutch does not have as much torque capacity as expected. Such a discrepancy may be the result of clutch degradation, incorrect sensor readings, or an error in a clutch torque estimation algorithm. In such an example, when transmission assembly input torque is increased above clutch torque capacity, the transmission assembly input speed may increase above the transmission input shaft speed, which may result in clutch slip.
The inventors herein have recognized these issues, and have developed systems and methods to at least partially address the above issues. In one example, a method is provided, comprising transferring transmission input torque through a clutch of a dual clutch transmission controlled to a first capacity less than a maximum capacity, and in response to a desired transmission input torque exceeding the capacity, increasing torque output of a motor coupled downstream of the dual clutch transmission to assist in meeting a wheel torque demand, while maintaining transmission input torque below the first capacity. In an example, the motor coupled downstream of the dual clutch transmission includes an electric machine configured to provide torque to driven wheels, where driven wheels include one or more wheels receiving power from the engine, or one or more electric motors coupled to non-driven wheels. In this way, requested wheel torque may be met and without resultant clutch slippage.
In one example, the method may comprise increasing the clutch capacity from the first capacity to a second capacity greater than the desired transmission input torque while the torque output of the motor is assisting in meeting wheel torque demand. In such an example, the method may further comprise increasing transmission input torque while increasing the clutch capacity to the second capacity, while maintaining the transmission input torque below the increasing clutch capacity. Such a method may further include reducing output of the motor while increasing transmission input torque, to meet the wheel torque demand. Still further, the method may comprise increasing engine torque to the desired input torque while offsetting the increased engine torque via negative torque provided via an integrated starter/generator coupled to the engine, where increasing the transmission input torque while increasing the clutch capacity is accomplished via reducing the negative torque provided via the integrated starter/generator.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.